Steam generator with split flow preheater

ABSTRACT

A shell and tube steam generator having a U-shaped or straight tube bundle and a preheater. The preheater is disposed on the cold leg portion of the tube bundle and comprises walls and a plurality of baffles which cooperate to cause the feedwater entering the preheater to split and follow two flow paths, one in a generally upward direction and the other in a generally downward direction. The preheater has an outlet disposed to cause the fluid flowing therefrom to flow across the tube sheet and then upwardly along the outside surface of the tubes to optimize the temperature and pressure of the steam produced and to eliminate the channeling of cold fluid along the tubes which causes thermal stresses and shock on the tube sheet and lower shell.

United States Patent [191 Byerley et al.

[451 July 29,1975

[ STEAM GENERATOR WITH SPLIT FLOW PREHEATER [75] Inventors: Wilbur M. Byerley; Robert R.

Bennett, both of Tampa, Fla.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[221' Filed: July 24,1972

2! Appl. No.: 274,698

3,706,301 l2/l972 Penfield,.lr ..l22/32 Primary Examiner-Kenneth W. Sprague Attorney, Agent, or FirmD. C. Abeles [5 7 ABSTRACT A shell and tube steam generator having a U-shaped or straight tube bundle and a preheater. The preheater is disposed on the cold leg portion of the tube bundle and comprises walls and a plurality of baffles which cooperate to cause the feedwater entering the preheater to split and follow two flow paths, one in a generally upward direction and the otherin a generally downward direction. The preheater has an outlet disposed to cause the fluid flowing therefrom to flow across the tube sheet and then upwardly along the outside surface of the tubes to optimize the temperature and pressure of the steam produced and to eliminate the channeling of cold fluid along the tubes which causes thermal stresses and shock on the tube sheet and lower shell.

2 Claims, 9 Drawing Figures SHEET PATENTED JUL 2 9 I975 TUBE LENGTH TTW'JCS PATENTED JUL 2 91975 SHEET N 000 8 9 m 6 500 F pm k 1 STEAM GENERATOR WITH SPLIT FLOW PREHEATER BACKGROUND OF THE INVENTION This invention relates to steam generators for nuclear power plants and more particularly to stlch steam generators having a preheater portion disposed therein.

In a steam generator utilizing a heated fluid to produce steam. the pressure of the steam is a function of the logarithmic means temperature difference (LMTD) of the two fluids. Thus, by increasing the LMTD, the pressure of the steam can be increased.

An obvious way to increase the LMTD is to raise the temperature of the primary fluid, however. in nuclear steam generating systems the temperature of the primary fluid is a limiting factor in the design of the system and is normally set at a maximum allowable safe value.

Increasing the LMTD may also be accomplished within the steam generator by providing a preheater chamber in which feedwater is heated to a temperature lower than the boiling point. However, one problem associated with preheaters is channeling of cold feedwater to the tube sheet which results in. t-hermal stresses when the cold feedwater contacts the lower shell and tube sheet.

SUMMARY OF THE INVENTION In general, a steam generator, when made in accordance with this invention, utilizes a U-shaped bundle of tubes to provide the heat transfer surface required to transfer heat from a first fluid to vaporize a second fluid. The tube bundle is so disposed within the steam generator that the first fluid flows through the tubes, thereby creating a hot leg portion and a cold leg portion in the tube bundle. The steam generator comprises a shell, a tube sheet in which the ends of the tubes are fastened, a head secured to the tube sheet and forming a header for the tubes, a fluid inlet nozzle for the second fluid and a preheater disposed within the shell. The preheater is cooperatively associated with a portion of the cold leg and the second fluid inlet nozzle, and has walls and baffles so disposed to cause the second fluid flowing therethrough to split into two flow streams, one flowing generally upwardly and the other flowing generally downwardly, whereby the downwardly flowing stream makes a plurality of passes across the lower portion of the cold leg and prevents cold second fluid from contacting the tube sheet.

BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of this invention will become more apparent from reading the following detailed description in connection with the accompanying drawings, in which:

FIG. 1 is a vertical sectional view of a steam generator having a preheater made in accordance with this invention;

FIG. 2 is an enlarged partial sectional view showing the preheater in more detail;

FIG. 3 is a sectional view of another steam generator utilizing a preheater made in accordance with this invention;

FIG. 4 is a sectional view taken on line IVIV of FIG. 3;

FIG. 5 is a sectional view taken on line VV of FIG.

FIG. 6 is a sectional view of another steam generator incorporating a preheater made in accordance with this invention;

FIG. 7 is a sectional view taken on line VII-VII of FIG. 6;

FIG. 8 is a sectional view taken on line VIllVIll of FIG. 6; and

FIG. 9 is a graph showing the temperature at various locations along the tubes in the steam generators.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings in detail, FIG. I shows a steam or vapor generator 1, which utilizes a U-shaped bundle of tubes 3 to provide the heat surface required to transfer heat from a first fluid to vaporize or boil :1 second fluid. The steam generator 1 comprises a vessel 5 having a vertically disposed tubular shell portion 7 and an end closure or head 9 enclosing one end of the shell, the upper end, and a channel head 11, enclosing the other end of the shell, the lower end. A tube sheet 13 is madeintegral with the channel head II and has a plurality of holes 14 disposed to receive the ends of the U-shaped tubes 3. A divider plate 15 is centrally disposed within the channel head 11 and divides the channel head into two compartments l7 and 19, which serve as headers for the tubes 3. The compartment on the left, as shown in FIG. 1, is the first fluid inlet compartment l7 and has a first fluid inlet nozzle 21 in communication therewith. The compartment on the right, as shown in FIG. 1, is the first fluid outlet compartment 19 and has a first fluid outlet nozzle (not shown) in communication therewith, thus causing the primary inlet fluid to flow through the tubes and thereby create a hot leg portion 23, the portion shown on the left in the drawings, and a cold leg portion 25, the portion shown on the right in the drawings. A second fluid or feedwater inlet nozzle 27 is disposed in the lower portion of the shell 7 adjacent the tube sheet 13 and has a thermal sleeve 29 disposed therein.

A preheater 31 is disposed within the shell 7 adjacent the tube sheet 13. The preheater 31 has walls 33 and 35 and baffles 37 and 39, which are cooperatively associated with a portion of the cold leg 25 of the tube bundle 3 and the feedwater inlet nozzle 27 to cause the influent feedwater to split as it enters the preheater and follow two generally sinuous paths through the preheater. In one of the paths, the flow is generally counter to the first fluid and generally upward through {the preheater, while in following the other path the fluid flows generally downward and concurrent or parallel to the flow of first fluid. As the feedwater is discharged from the downwardly flowing portion of the preheater 31 it sweeps the tube sheet 13 and then flows upwardly over the outer surface of that portion of the tubes not incorporated in the preheater.

The walls of the preheater 31 are formed from a plate 33 and a lower portion of a wrapper 41 which enwraps the tube bundle. The wrapper 41 provides an annular space 43 adjacent the shell to reduce thermal shock on the shell.

The baffles 37 are so disposed in the upper portion of the preheater to provide a greater mass flow rate than flows in the lower portion of the preheater. As shown in the drawings, the baffles 37 are spaced apart a greater distance than the baffles 39 causing approximate percent of the feedwater to flow generally upwardly and counter current to the flow of the first fluid.

The thermal sleeve 29 is disposed within the second fluid nozzle 27 and provides an annular space between the sleeve 29 and the nozzle 27 to prevent rapid temperature gradients, which produce thermal shocks and stresses. The sleeve 29 is also seal-welded to the wrapper 41 and the nozzle 27 to prevent the relatively cold influent feedwater from contacting the shell or the tube sheet before passing through the preheater.

Disposed vertically within the preheater adjacent the sleeve is an impingement plate 45 which prevents the high velocity influent feedwater from impinging on the tubes 3. The steam generator also has means for separating water from steam disposed in the upper portion of the shell, which has an enlarged cross section. For a more complete description of this portion of the steam generator reference may be made to the application of R. R. Bennett Ser. No. 224,804 filed Feb. 9, 1972 and assigned to the same assignee, which application is hereby incorporated by reference. The annular space 43 between the wrapper 41 and the shell 7 provides a recirculation path for water removed by the moisture separating means disposed in the top of the vessel 5. This water flows around the thermal sleeve 29 and prevents excessive thermal shocks in the second fluid inlet nozzle 27. This water also mixes with the flow from the lower portion of the preheater and the combined temperature of these two fluids is sufficiently subcooled to prevent'boiling under the preheater, but is hot enough so that boiling is initiated once the combined fluid begins to flow upwardly, when the velocity is reduced. The mixing of the water from the moisture separator and the discharge from the lower portion of the preheater cooperate to enter the tube bundle from 360 inhibiting boiling under the preheater and providing high cross flow velocities, which sweep the tube sheet to prevent chemical hideout and the formation of steam pockets adjacent the tube sheet 13.

The steam generator 1 shown in FIG. 3 incorporates the same U-shaped tubes 3, channel head 11 and integral tube sheet 13, however, a second fluid outlet nozzle 51 is shown. The shell 7 is generally the same diameter throughout its length and a divider plate 53 extends upwardly from the tube sheet 13 and cooperates with the wrapper 41' to form a steam or vapor enclosure 54 forming a superheater portion around the hot leg 23 portion of the tube bundle 3. The thermal sleeve 29 cooperates with baffles 55 to distribute the inlet fluid or feedwater to the preheater so that it splits into two flow paths one generally upwardly and counter current to the flow of the first fluid and the other generally downwardly and parallel or concurrent with the flow of the first fluid. The portion of the feedwater which flows downwardly, sweeps the tube sheet 13, and then flows upwardly in the annular space 43' between the wrapper 41' and the shell 7, and then rejoins the effluent feedwater from the upper portion of the preheater and then flows upwardly over the remaining portion of the cold leg where the feedwater boils or is vaporized to produce steam. As the second fluid reaches the upper end of the cold leg 25, it becomes saturated vapor or steam. The saturated steam then flows downwardly through the superheater portion or vapor enclosure 54 and becomes superheated steam before leaving the steam generator through the second fluid outlet nozzle 56.

As shown in FlGS. 3, 4 and 5, baffles 57 and 58 are utilized in the superheater portion to cause the steam to follow a sinuous path as it flows in a generally downward direction through the superheater to the second fluid outlet nozzle 56.

The steam generator shown in FIG. 6 is similar to the one shown in FIG. 3, except the dividing plate 61 is disposed within the hot leg portion 23 of the tube bundle and tubes are omitted adjacent the second fluid inlet and outlet nozzles 27 and 56 respectively to provide a flow path for the second fluid and to provide space for the influent feedwater to expand and reduce its velocity prior to contacting the tubes 3. The dividing plate 61 has an arcuate portion 63 forming the upper end thereof.

The wrapper 41" generally has the same shape as the tube bundle and comprises two arcuate plates 65 seal welded to two parallel flat plates 67 and 69 and a spherical end closure or heat portion 70.

As shown in FIGS. 6, 7 and 8, the preheater 31" is closed by a portion of the wrapper 41" including the arcuate plates 65 and a lower portion of the flat plate 69 which extends outwardly and is seal welded to the shell 7" adjacent the preheater 31".

The baffles 37" and 39" are adapted to cause the influent feedwater to split so that a portion flows generally upwardly and counter current to the flow of the first fluid and the other portion flows generally downwardly through the preheater. An upper outlet port 71 directs the effluent feedwater from the preheater to an annular space 72 between the plate 69 and the shell 7" and then in a generally downward direction where it joins the effluent feedwater flowing from the downwardly flowing portion of the preheater. The combined flow sweeps the tube sheet and then flows generally upwardly and boils or vaporizes so that saturated steam or vapor enters the enclosure formed by the wrapper 41" and the divider plate 61. The second fluid increases in temperature as it flows to the second fluid outlet nozzle 59 and becomes superheated steam.

FIG. 9 illustrates the thermal performance of the steam generators hereinbefore described compared with a steam generator not incorporating a preheater. The dashed-dotted lines 101 and 102 represents the steam generator without a preheater. The solid lines 103 and 104 represent the steam generator with a preheater, but without a superheater. The'dotted lines 105 and 106 represent a steam generator with a preheater and a superheater.

The preheaters hereinbefore described, advantageously provide multi-pass cross flow over a portion of the cold leg 25 with sufficient velocity to preclude boiling within the preheater 31 and insure stability of the unit, however, when the velocity of the feedwater is reduced, when the feedwater enters the remainder of the steam generator, boiling occurs immediately. Although boiling occurs over a reduced area, the increase in the feedwater temperature and the LMTD vastly overshadows the pressure loss in the preheater 31 and the loss due to the reduction in the heat transfer area allotted to boiling. This is a result of the very high LMTD in the preheater, which produces a more efficient utilization of the heat transfer surface in the tube bundle. The pressure drop in the preheater 31 advantageously limits the effect of operating pressure fluctuation and adds stability, while the split flow is a compromise between maximum heat transfer capabilities and maximum reliability. Providing counterflow of the feedwater and the first fluid would optimize the heat transfer in the preheater, however, such arrangement also allows a small quantity of cold feedwater to channel along the tubes and contact the tube sheet and induce thermal shock and stresses which result in failure of the tube sheet.

What is claimed is:

l. A vapor generator having a plurality of U-shaped tubes forming a tube bundle for providing the heat transfer service required to transfer heat from a first fluid to vaporize and superheat a second fluid, said tube bundle being so disposed within said vapor generator that the first fluid flows through the tubes creating a hot leg portion and a cold leg portion, said vapor generator comprising a tube sheet for receiving the ends of the tubes; a shell portion cooperatively associated with said tube sheet; a wrapper disposed within the shell portion and generally enclosing said tube bundle; a dividing plate cooperatively associated with said wrapper to form an evaporator enclosure incorporating at least a portion of the cold leg portion of the tube bundle and a vapor enclosure incorporating at least a portion of the hot leg portion of the tube bundle; a plurality of baffles disposed within said vapor enclosure causing the fluid flowing therethrough to follow a sinuous path over the outer surface of said portion of the hot leg; a preheater enclosing at least a portion of the cold leg portion of said tube bundle; a fluid inlet nozzle for the second fluid disposed in fluid communicationwith said preheater; and a fluid outlet nozzle for the second fluid disposed in fluid communication with the vapor enclosure. whereby the second fluid entering the preheater is heated to approximately saturation temperature. flows through the evaporator portion. wherein the second fluid is vaporized, and enters the vapor portion, wherein the vaporized fluid is superheated prior to being discharged through said second fluid outlet nozzle.

2. A steam generator as set forth in claim 1, wherein the wrapper forms an annular chamber adjacent the shell and the preheater has a plurality of baffles which split the influent fluid so that one portion thereof flows downwardly through the preheater, sweeps the tube sheet, flows upwardly into the annular chamber and then to the evaporator portion of the tube bundle and the other portion of the influent fluid flows upwardly through the preheater and then into the evaporator portion of the tube bundle. 

1. A vapor generator having a plurality of U-shaped tubes forming a tube bundle for providing the heat transfer service required to transfer heat from a first fluid to vaporize and superheat a second fluid, said tube bundle being so disposed within said vapor generator that the first fluid flows through the tubes creating a hot leg portion and a cold leg portion, said vapor generator comprising a tube sheet for receiving the ends of the tubes; a shell portion cooperatively associated with said tube sheet; a wrapper disposed within the shell portion and generally enclosing said tube bundle; a dividing plate cooperatively associated with said wrapper to form an evaporator enclosure incorporating at least a portion of the cold leg portion of the tube bundle and a vapor enclosure incorporating at least a portion of the hot leg portion of the tube bundle; a plurality of baffles disposed within said vapor enclosure causing the fluid flowing therethrough to follow a sinuous path over the outer surface of said portion of the hot leg; a preheater enclosing at least a portion of the cold leg portion of said tube bundle; a fluid inlet nozzle for the second fluid disposed in fluid communication with said preheater; and a fluid outlet nozzle for the second fluid disposed in fluid communication with the vapor enclosure, whereby the second fluid entering the preheater is heated to approximately saturation temperature, flows through the evaporator portion, wherein the second fluid is vaporized, and enters the vapor portion, wherein the vaporized fluid is superheated prior to being discharged through said second fluid outlet nozzle.
 2. A steam generator as set forth in claim 1, wherein the wrapper forms an annular chamber adjacent the shell and the preheater has a plurality of baffles which split the influent fluid so that one portion thereof flows downwardly through the preheater, sweeps the tube sheet, flows upwardly into the annular chamber and then to the evaporator portion of the tube bundle and the other portion of the influent fluid flows upwardly through the preheater and then into the evaporator portion of the tube bundle. 